The present invention relates to the field of telescope mirror fabrication and more particularly to a method for reducing the cracking of ribbed mirrors during thermal processing.
There is a great motivation to use ribbed mirrors in telescopes. Ribbed mirrors are lightweight, less glass is needed and commonly available thin glass can be utilized. Ribbed mirrors are stiffer than thin mirrors, which simplifies the supporting structure and reduces dynamic disturbance and vibration. Ribbed mirrors are cheaper to produce, cheaper to launch into orbit, and can be aimed more quickly with less power. Moreover, astronomical telescopes that operate on earth cannot produce sharp images until the mirror surface cools to nearly the same temperature as the ambient air. Ribbed mirrors cool more rapidly than thick mirrors.
George Ritchey, who fabricated the Mt. Wilson 100, a solid mirror, experimented with ribbed mirrors in France in the late 1920's. The ribs were bonded between flat faceplates with Bakelite, the best adhesive then available.
Possibly the first large ribbed mirror is the Palomar 200. Unlike the Ritchey mirrors, the Palomar ribbed mirror has an open back and is a single casting. Subsequently, many large telescopes have been built with ribbed mirrors. In 1980 the Hubble Space Telescope primary mirror was completed. Like the Ritchey mirrors, it has a square rib pattern. It is made of fused silica front, back, and rib pieces welded at high temperature. In Sky & Telescope Magazine, July 1984 p. 71, Ric Rokosz taught the world how to make ribbed mirror blanks by fusion welding of separate pieces of glass. He used Pyrex low expansion glass and high temperatures. As a result, his mirror structure became very soft and required internal support during the fusing process. It has subsequently been found that satisfactory fusion welding can take place at lower temperatures and that, consequently, internal support is not required if the rib pattern resists relative rotation and translation of the faceplates.
Unfortunately, mirrors are exposed to heat loads during optical polishing and during application of some kinds of coating. If the mirror is not made of an expensive material with low coefficient of expansion, thermal gradients will result in mechanical stress that can crack the glass. Because glass is a poor heat conductor, applied heat loads (especially transient heat loads) lead to large thermal gradients—especially where heat flows around a sharp corner.
Development of a fabrication method which can conduct the heat away from the glass during pitch lapping and application of thermally cured coatings represents a great improvement in the field of ribbed mirror fabrication and satisfies a long felt need of telescope builders.